DIGITAL CPFSK TRANSMITTER AND NONCOHERENT RECEIVER/DEMODULATOR IMPLEMENTATION

Abstract

As radio frequency communications continue to grow, the available frequency spectrum becomes a valuable commodity, and as such may best be utilized in smaller and smaller increments. Hence, bandwidth efficient modulation schemes are desirable in that they allow better use of the frequency spectrum. Previous research has considered the spectral efficiency characteristics of several modulation schemes. It can be shown that eight- and 16-level Continuous Phase Frequency Shift Keying (CPFSK) can achieve 2 bits/s/Hz spectral efficiency packing density with an Eb/No of 12 dB specified at a 10^(-5) bit error rate [2]. In addition to its spectral characteristics, CPFSK possesses two other appealing characteristics. First, CPFSK maintains a constant amplitude signal, which is appropriate for nonlinear channels, as it will experience fewer adverse effects than a non-constant envelope signal. Hence, a nonlinear high power amplifier in the signal path is acceptable. Second, the information in a CPFSK signal can be retrieved via non-coherent demodulation, which is appropriate for multipath fading channels. This paper describes a low cost implementation of a CPFSK transmitter and noncoherent receiver based around the Motorola DSP56002 (56K) digital signal processor. In addition to the features associated with CPFSK, this implementation requires little power and is physically small and thus is suitable for many telemetry applications.